Pope Francis, the first Latin American pontiff in history, today meets with a familiar face -- in fact, a familiar foe: Argentine President Cristina Kirchner, with whom he has clashed in the past over a slew of social issues.

As then Cardinal Jorge Mario Bergoglio, the archbishop of Buenos Aires, Francis was at odds with Kirchner in Argentina as she successfully fought to legalize gay marriage in recent years. Defying the church, Kirchner adopted a series of measures such as mandatory sex education in schools, free distribution of contraceptives in public hospitals, and the right for transsexuals to change their official identities.

The two compatriots' lunchtime meeting -- Francis' first with a head of state -- will take place at the Santa Marta residence in Vatican City.

Kirchner is one of many world leaders in Rome for the new pontiff's inaugural mass Tuesday morning. Vice President Biden, representing the United States, arrived Sunday evening. In his first five days as the new leader of the Roman Catholic Church, Francis has won fans around the world with his gestures of informality, humility and humor.

While Francis has changed some of the formalities and stiffness that his predecessor Pope Emeritus Benedict XVI brought to the papacy, Rob Mickens, Rome correspondent for The Tablet, warned in an interview with ABC News that Francis will not change the crux of the church's teachings.

"He is a radical and I think he is going to be a bit hard-nosed," Mickens said. "Doctrinally, married priests, women priests, gays and lesbians, I don't expect any change on those core teachings that have been part of the Vatican lexicon for the last 30 years."

In recent days Francis has come under fire for his actions during his homeland's "Dirty War" decades ago. From 1976 to 1983, a military junta ruled the country, allegedly making up to 30,000 political opponents "disappear." Years later war criminals testified that catholic priests and chaplains blessed torture chambers and absolved troops of their sins after they had thrown dozens of dissidents from a plane into a river. Francis has faced accusations from human rights activists that as a young Jesuit provincial leader he turned a blind eye to the incarcerations and murders of the junta's leftist rivals.

"Before they elected the pope, before we knew who was going to be pope, I said the cardinals are very, very aware that they need to elect someone that doesn't have skeletons in his closet," Mickens said. "Whether they made a good choice on that regard I don't know."

The Vatican last Friday forcefully denied the allegations against the new pope.

"There has never been a credible accusation against him," Vatican spokesman Rev. Federico Lombardi told reporters, adding that the church's new leader in fact tried to protect many people from the military junta and the criticism of his actions during that time stem from "anti-clerical" and "left-wing" elements.